1. Field of the Invention
The present invention relates to a racket frame and more particularly to a lightweight racket frame whose vibration-damping performance is enhanced and vibration-damping factor can be adjusted easily without using a heavy dynamic damper.
2. Description of the Related Art
In recent years, females and seniors demand a racket which hits a ball a long distance by applying a small power thereto. For example, a so-called “thick racket” which is thick in the out-of-plane direction (ball-hitting direction) in the ball-hitting plane of the racket frame is popular in a wide range of ages as well as females and seniors because the “thick racket” hits a ball a longer distance than conventional rackets. Therefore in recent years, rather than metal or wood, fiber reinforced resin which is lightweight, has a high specific strength, and has a high degree of freedom in design is used as a popular material of the racket frame. Thermosetting resin reinforced with carbon fiber or the like having a high strength and a high elastic modulus is popular as the fiber reinforced resin.
When the racket frame is lightweight by making it of this kind of fiber reinforced resin, the repulsion for a ball deteriorates because a kinetic energy to be transmitted to a ball at an impact time decreases. Further the vibration of the racket frame and an impact generated when the ball is hit are transmitted readily to a player's hand.
Recently a lightweight racket having a weight not more than 280 g is manufactured. When a lightweight racket having a weight not more than 250 g is designed, the racket is demanded to be lightweight and have high vibration-damping performance and shock-absorbing performance.
To improve the vibration-damping performance of the racket frame, various constructions having a dynamic damper mounted on the racket frame have been proposed.
For example, in the tennis rackets disclosed in Japanese Examined Patent Publication No. 52-13455 (patent document 1), as shown in FIG. 11, the cantilevered dynamic damper composed of the long and narrow elastic material is installed at the end of the grip. The base of the steel wire 2 having a weight 1 mounted on its front end is embedded in the racket frame.
In this construction, the dynamic damper is extended from the rear end of the racket. Thus the dynamic damper interferes with a player swinging a racket.
In Japanese Patent Application Laid-Open No. 2000-24140 (patent document 2), as shown in FIG. 12, the cap 6 to be mounted at the free end of the grip part 5 disposed at the position of the antinode of the amplitude of various vibration mode is improved. That is, a vibration-absorbing apparatus 6 whose vibration weight 6b is accommodated in the cavity 6a of the cap 6 is mounted at the free end of the grip part 5.
This construction contributes to damping of various vibration modes of the racket frame. Because the natural frequency is adjusted by the auxiliary weight, the weight of the vibration-absorbing apparatus increases much. Further the construction of the cap 6 makes it difficult to form a construction having a cover for the grip end. Thereby broken pieces generated in the hollow portion of the grip part tend to generate a sound.
In Japanese Patent Application Laid-Open No. 2003-164548 (patent document 3), as shown in FIG. 13, the balance-adjusting metal 8 is bonded to the concave groove 5a formed on the surface of the grip part 5 with a soft adhesive agent. The balance-adjusting metal 8 does not have a construction which easily vibrate, thereby having difficulty in resonating with various frequencies. As such the construction disclosed in the patent document 3 is incapable of providing a sufficient vibration-damping effect.
A construction is proposed in which the mass member is disposed at the head part of the racket frame. This construction causes the racket frame to be considerably heavy and the balance thereof to be large. Thus the racket frame has a low operability. A construction is proposed in which a vibration-absorbing member is disposed in a string hole. This construction causes the vibration-absorbing member to contact the string. Thus the vibration of the vibration-absorbing member is restricted. Consequently the vibration of the string is damped but the vibration of the racket frame cannot be damped sufficiently.
Patent document 1: Examined Patent Publication No. 52-13455
Patent document 2: Japanese Patent Application Laid-Open No. 2000-24140
Patent document 3: Japanese Patent Application Laid-Open No. 2003-164548